The windows (also called mirrors) constitute one of the weak points of power lasers. Thus, as soon as a particle deposits on them, it absorbs the radiation and retransmits it by conduction to the window. The local heating which results gives rise generally to the destruction of the window. It is thus considered that, under particularly careful conditions of cleanliness, 20-30 kW/cm.sup.2 constitutes a limit value of the radiation flux permissible on existing windows. In the case of CO.sub.2 lasers, in which the coefficient of reflection of the partially transmitting window is generally comprised between 50 and 90%, the transmitting windows are constituted by a substrate which is generally ZnSe, if desired AsGa, with multilayer coatings ensuring a high coefficient of transmission, the reflecting windows being generally constituted by a substrate of treated germanium or metallic mirrors, particularly of copper, with protective treatment.